Slip resistant horizontal semiconductor wafer boat

ABSTRACT

A horizontal wafer boat for maintaining semiconductor wafers during wafer processing is disclosed. The wafer boat is configured to reduce the likelihood wafer slip when wafers are heated to processing temperatures of about 1000° C. and above.

FIELD OF THE INVENTION

The invention is in the field of equipment used in the processing andmanufacturing of semiconductor wafers. More particularly, the presentinvention relates to an improved horizontal wafer boat.

BACKGROUND OF THE INVENTION

In the manufacture of semiconductors, silicon wafers are thermallyprocessed. One method to process the wafers is to use horizontal furnacetubes. The wafers are processed to change their electrical propertiesand to build circuits. The temperatures for these processes range from600 to nearly 1400° C.

The production of semiconductors is a very controlled process. As partof this process, furnace operations are performed on silicon wafers tobuild layers on the wafer and to dope materials into the wafer to changeits electrical properties. Discrete dielectrics and pathways are formedto create capacitors and transistors. With precise configuration, adevice is created.

Furnacing operations are generally classified into two categories,Atmospheric and Low Pressure Chemical Vapor Deposition (LPCVD).Atmospheric operations are used to anneal, to diffuse dopants intowafers, or to form oxide layers. These processes are typically performedat high temperatures, i.e., greater than about 900° C. Some atmosphericoperations for high purity or for deep diffusion can reach temperaturesof 1350° C.

LPCVD operations are used to build layers of polysilicon or siliconnitride onto the wafer. These operations take place under a partialvacuum, typically at lower temperatures in a range of between about 600°C. and 900° C.

A combination of the operations described above is used to construct athree dimensional device on the wafer. A simple device such as a powerchip can have two layers while a complex logic circuit might have morethan seven layers.

There are several different methods of furnacing wafers, referred to ashorizontal, vertical, and Rapid Thermal Processing (RTP) methods. RTP isa single wafer process, whereas both vertical and horizontal furnacingare batch processes. More specifically, horizontal furnacing refers to aprocess in which many wafers are positioned on a wafer holder, or“boat”, which is inserted into a horizontal furnace tube. Devices, suchas wafer boats (also known as conti boats), which are subjected tofurnace operations during semiconductor wafer processing are referred toherein as “furnaceware”.

The material used to form the wafer boat must be resistant to hightemperatures and not introduce impurities into the operation. Whensilicon wafers were first processed, the support fixtures were primarilymade out of quartz. Quartz, however, suffers from several drawbacks whenused in the production of wafer boats. In particular, at temperaturesabove about 1000° C., quartz tends to creep. After repeated furnacecycles, the quartz boats deform to an unacceptable degree. Wafertransfer operations are typically automated and it is critical to insertthe wafer into a well defined feature of the furnaceware. If there ismisalignment, the wafers may “crash” into the furnaceware contaminatingthe entire wafer load and often breaking. Broken wafers can introduceparticles into the clean room environment and affect other processes aswell.

Another drawback is particle generation. During LPCVD operations, alayer of deposited material, such as silicon nitride, is built up ontothe surface of the wafer. The material that forms on the wafer duringLPCVD operations, along with the wafer itself, have comparable thermalexpansion rates such that a good mechanical and chemical bond exists.LPCVD deposits do not, however, adhere well to quartz because ofmismatched thermal expansion coefficients which produce stress on thelayers when the furnaceware is subjected to temperature changes. Thisstress causes the layers to flake and can introduce device-damagingparticles into the system.

Chemical etching can also cause problems with quartz. Furnaceware iscleaned on a regular cycle to remove layers which have developedthereon. Typically, acid baths are used to remove these layers. Quartzcan be chemically etched by the cleaning solutions used, and this cancause quartz furnaceware to lose strength and dimensional stability.

Many of the disadvantages known to exist in quartz furnaceware can beavoided by substituting other materials, such as silicon carbide (SiC)in the place of quartz. SiC has a thermal expansion coefficient similarto LPCVD depositions which form a mechanical and chemical bond to SiC.One useful replacement material is recrystallized SiC, available fromSaint-Gobain Industrial Ceramics of Worcester, Mass., under thetradename CRYSTAR®. This material is a silicon carbide ceramic which hasbeen impregnated with high purity silicon metal. Due to its robustmechanical properties through a wide range of temperatures and puritycharacteristics, CRYSTAR® ceramic has been shown to be an excellentalternative to quartz. CRYSTAR® may be used to support wafers duringfurnacing operations and can also serve as the furnace chamber.

As wafer sizes have increased, and as the size of features on the wafershas decreased, technological improvements in photolithograpy,inspection, furnacing, clean rooms and other areas have been required.Current production facilities for semiconductor devices employ wafersranging in diameter from 100 mm to 200 mm. Currently, there is a desireto process wafers as large as 300 mm in diameter because such wafersizes will allow more than twice as many chips or dies to be fabricatedon each wafer. This is desirable because fine feature size devicesrequire critical processing parameters, and there is a strong desire toreduce the number of wafers being processed and have a more tightlycontrolled production environment. Unfortunately, the shift to 300 mmwafers has been a slow process due to the large number of technicalproblems associated with the handling of large wafers.

Horizontal wafer manufacturing involves a series of thermal processingsteps to build the device. Horizontal processing temperatures in excessof about 1000° C. for at least some of the manufacturing steps cannot beavoided. Due to the high temperatures and wafer stress, wafer slip canoccur. A wafer is a single crystal disk. Wafer slip is the permanentplastic deformation of the wafer's crystal lattice. The transitiontemperature from brittle to ductile behavior of the wafer is about 720°C. Therefore, slip can occur at process temperatures above 720° C.

Wafer slip is important to the manufacturer because it has a negativeimpact on device performance. A device is made up of a series of gateswhich can switch states. These gates have precise features which must bemaintained. If a slip plane occurs, the gates can become corrupted andfail to function properly. Even if the slip plane is small, deviceperformance can be compromised because the dielectric properties havebeen changed. Typically, rather than investing more time in processing,a wafer with slip is scrapped.

Wafer slip creation is affected by several factors includingtemperature, gravitational stress, thermal stress, wafer type, waferdefects (edge chips, existing dislocations, oxygen content), andprevious processing steps. As the thermal energy within the waferincreases, the energy required to induce slip decreases. Once the 720°C. ductile threshold is reached, slip lines can generate relativelyeasily. Shear stress, the dominant contributor, acts in plane and pushesthe lattice so that it dislocates. Experimental and theoretical studieshave analyzed the effects of temperature on allowable shear stress.Allowable shear stress is defined to be the maximum stress before theonset of slip.

Wafer mechanical stress and thermal differences are directly affected bythe wafer boat. Accordingly a need exists for a wafer boat thatminimizes these stresses with the resultant minimization of wafer slip.

The prior art discloses wafer boats for use in the manufacture andprocessing of semiconductor wafers. However, unlike the presentinvention, the prior art does not address nor overcome the disadvantagesnoted hereinabove. For example, in JP6124911, there is disclosed ahorizontal wafer boat having a plurality of slots wherein a plurality ofwafers are mounted. Each slot of the wafer boat, however, comprises aplurality of what appear as stabilizing or support members referred toas “wafer falling-down preventing members” which are provided withgrooves to prevent the wafers from falling. This wafer boat alsocomprises flat supporting parts to support the weight of the wafer withits flat portion positioned in the slot. The wafer falling-downpreventing members and the flat supporting parts, however aredisadvantageous to the wafer positioned in the slot since the pluralityof wafer falling-down preventing members and the flat supporting memberscontribute to wafer stress and wafer slip. Additionally, the boatdescribed in JP6124911 is constructed to accommodate wafers with a flatportion positioned in the slot. This positioning of the wafer in theslot with its flat portion being supported by the boat allowsside-to-side movement of the wafer in the slot, such as during transferof the boat to and from a furnace. This wafer motion places stress onthe wafer, increases particle formation and thus lowers device yields.

Thus, a need exists for a wafer boat which eliminates the disadvantagesof the prior art wafer boats.

SUMMARY OF THE INVENTION

The wafer boat of the present invention is configured to reduce thelikelihood wafer slip when wafers are heated to processing temperaturesin excess of about 1000° C. The inventive wafer boat includes two uppersupport guides to maintain the wafer in the vertical orientation, and alower, supporting groove to support the weight of the wafer. Thematerial of the boat is selected, and the groove is shaped, such thatwhen the wafer and boat are subjected to wafer processing temperaturesof about 1000° C. and above, the shape of the groove will substantiallycorrespond to the shape of the wafer contacting the groove, therebysupporting the wafer across the entire arc over which the wafer iscontacted by the groove.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a horizontal wafer boat loadedwith wafers;

FIG. 2 is a schematic representation of a wafer boat of the presentinvention viewed from one end;

FIG. 3 is a schematic representation of a wafer boat of the presentinvention viewed at an oblique angle from above;

FIG. 4 diagrams the angle α for determining one relevant dimension ofthe inventive wafer boat; and

FIG. 5 plots the radius of the inventive boat and the radius of thewafer as a function of temperature.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a horizontal, silicon carbide waferboat configured to reduce wafer slip. Several factors have beenconsidered in the development of the wafer boat of the presentinvention. In particular, the present invention recognizes the followingprinciples: a) silicon strength decreases significantly at temperaturesabove about 600° C. and dramatically at temperatures above about 900°C.; b) below about 720° C., silicon is brittle and will not form a slipplane; c) shear stress is the dominant stress which induces slip; and d)the total wafer stress must be below the slip generation stressthreshold.

Wafer mechanical stress may arise from three sources—gravity, waferpinching, and wafer-boat frictional forces. Wafer pinching andwafer-boat frictional forces are the result of differences between theboat and wafer thermal expansion. All of these forces are directlyrelated to the wafer boat design. As shown in FIG. 1, in a prior arthorizontal wafer boat 10, the wafers 12 are positioned in parallel,essentially vertical planes. Each wafer 12, which is generally circularin shape, is offset from each other at a constant spacing. The wafers 12are disposed in the wafer boat 10 at a predetermined pitch to preventthem from tipping. Due to handling and thermal concerns larger wafersrequire longer pitches. For wafer diameter up to 150 mm, the optimalpitch is 2.38125 mm, and for wafer diameter up to 200 mm, the optimalpitch is 4.7625 mm.

In the horizontal wafer boat 10 shown in FIG. 1, the wafer 12 fits intoprecision machined slots 14. The wafers 12 are supported by bottomsupport members 16 a, 16 b and guided by upper support members 18 a, 18b. The slots 14 prevent the wafer from tipping and contacting otherwafers or becoming significantly out of the vertical plane. The gravityreaction force at each support has been found to be equal to one half ofthe wafer weight. Consequently, the normal force at each support can beexpressed as:F_(normal)=0.5 (wafer weight)/cos(α)Thus, the cosine of the angle, α, is inversely proportional to the waferstress.

Another component of mechanical stress to analyze is wafer pinching.Wafer pinching occurs when the wafer is constrained by the wafer slotwhen it is thermally expanding. As the wafer load is inserted into thefurnace and processed, the wafers and the silicon carbide boat do notexpand at the same rate primarily due to the much greater mass of theboat. The wafers heat up much more quickly and therefore expandoutwardly. Thus, the boat must provide adequate space to allow for thethermal expansion difference between the wafer and the boat. The upperslots need to be designed with sufficient space to allow for thisexpansion. In addition, as the upper slots are positioned higher inrelation to the center of the wafer, more allowance must be given forwafer expansion. This wafer allowance is a function of both the lengthof the chord formed by the upper slots and the height of the upperslots. As such, an allowance for wafer expansion must be provided withregard to the upper slots and must increase as the height of the slotsincreases.

As noted above, because of its lower mass, the wafer is more thermallyresponsive than the boat, causing the wafer to expand and contract at adifferent rate than the boat. This mismatch in expansion means that thewafer must slide over the surface of the lower slots. As the waferslides over the support, static friction must be overcome to allow thewafer to move. The static friction creates tangential forces on thewafer. The friction force is a function of the normal force on the waferat the support points:F_(static friction)=μF_(normal)Substituting this relationship for the normal force yields:F _(static friction)=μ0.5 (wafer weight)/cosine αThe force needed to overcome static friction will also be affected bythe surface roughness and shape of the lower slot. The surface roughnessis relatively easy to control by good machining practices. A surfaceroughness of 1 to 2 micron Ra is achievable and has proven successful.The shape of the lower slot should not be a sharp point that could diginto the wafer and not allow smooth movement.

An additional property can be determined from the relationships above.To reduce friction effects, the wafer support angle should be minimized,the slot surface should be machined to less than 2 micron Ra, and thesupports should approximate a tangent or be continuous to the wafer.

An analysis of the heat transfer from the furnace to the wafers hasdemonstrated that radiation will be the dominant contributor to heattransfer at high temperatures. In other words, wafer areas that areexposed to radiation will heat up quickly.

In radiative heat transfer, there are source(s), target(s), andblocker(s). A source is defined as a mass at higher temperature whichgives off radiation. The target is defined as the mass which is beinganalyzed for heat transfer. The blocker is defined as a mass whichinterrupts the line of sight between a source and target.

Between the source and target surfaces, a geometric relationship calledthe radiation view factor exists. The radiation view factor defines howmuch of the radiant energy leaving the source actually hits the targetentity. Because of complex geometries including curved shapes, 100% ofthe target entity is not directly visible to 100% of the source entity.Since there is not always a direct line of sight, not all of theradiation energy is received by the target.

In a horizontal furnace system, the furnace tube is the source and thewafers act as the targets as well as the blockers. As the load heats up,the wafers also act as sources to each other and heat is transmittedfrom wafer face to wafer face bi-directionally through the wafer load.From an understanding of the different modes of heat transfer to thewafers, it becomes possible to define additional characteristics of theinventive wafer boat. The boat should maximize the line of sight betweenthe wafer faces and the furnace tube. Additionally, the boat designshould not have a large thermal mass which will significantly lag behindthe wafers thermally.

Wafer boats of the present invention are manufactured using slip castingtechniques and green machining. From the green state, the boats arefurnaced and then subjected to a final machining step. Priormanufacturing experience has shown that thin walled boats do not have ahigh rate of survival in this process. Also boats which have windows orcutouts close to the edge of the boat are prone to mechanical damageduring manufacturing and in subsequent use.

Considering the above effects, the boat wall nominal thickness should beno less than about 5 mm. Likewise, windows should be positioned aminimum of about 10 mm from the boat ends or any sharp transitions.

Taking the several considerations outlined above into account, theinventive wafer boat was developed. FIGS. 2 and 3 are schematicrepresentations of one embodiment of the wafer boat 30 in accordancewith the present invention, as viewed from different angles. Inparticular, unlike current wafer boats which provide support for eachwafer at four points (two lower points to support the weight of thewafer, and two upper points to maintain the wafer in the verticalorientation as illustrated in FIG. 1), the wafer boat of the presentinvention is provided with two upper support guides 36 a, 36 b tomaintain the wafer 32 in the vertical orientation, and a single lowersupporting grooved portion 38 to support the weight of the wafer 32.Once the material of which wafer boat 30 is fabricated is selected, thesupporting grooved portion 38 which is in a plane lower than the uppersupport guides 36 a, 36 b, is shaped having an arcuate configurationsuch that, when the wafer 32 and wafer boat 30 are subjected to waferprocessing temperatures of about 1000° C. and above, the shape of thesupporting grooved portion 38 will substantially correspond to the shapeof the part of the wafer 32 contacting the supporting grooved portion38, thereby supporting the wafer 32 across the entire arcuate portion ofa circular wafer's periphery which is in contact with the supportinggrooved portion. In other words, the lower arcuate periphery of thecircular wafer rests upon and is supported by the supporting groovedportion 38 when the wafer 32 is positioned in a slot 34 in wafer boat 30and maintained in a vertical position by the upper support guides 36 a,36 b. The wafer boat 30 having this configuration provides exceptionalsupport for and stabilization of the wafers 32 positioned in the slots34. Additionally, the wafer boat 30 of the present invention includesone or more large openings or windows 40 between each end of the boat inorder to increase the radiation view factors and decrease radiationblocking caused by the boat, as compared to boats currently known in theart.

Since they are formed of different materials, the wafer 32 and the waferboat 30 have different thermal expansion coefficients. In oneembodiment, the wafer boats of the present invention are formed of SiC.One preferred SiC comprises recrystallized SiC commercially availablefrom Saint-Gobain Industrial Ceramics Inc., of Worcester, Mass. underthe tradename CRYSTAR®. Such materials can comprise eitherrecrystallized SiC or silicon impregnated SiC where semiconductor gradesilicon has been used to fill porosity in the body. The siliconimpregnated material can be further provided with a layer of CVD-SiC toseal the surface and prevent silicon migration during use of the devicein wafer processing.

Recrystallized SiC, either impregnated or not, is preferred for waferboats as a result of its strength at high temperatures. Specifically,CRYSTAR® material has been found to be significantly stronger and moredimensionally stable than quartz throughout the semiconductor processingtemperature range. As a result, the material can be used to fabricatewafer boats that resist thermal distortion or sagging during theirworking lifetimes.

Boats formed from recrystallized silicon carbide (i.e., CRYSTAR®material) have been found to exhibit a thermal expansion coefficientthat is about 27% higher than that of polysilicon wafers. FIG. 5 showsthe radius of the inventive boat and the wafer as a function oftemperature. In effect, the CRYSTAR® recrystallized silicon carbide actsas a “smart” material, a material which transforms energy from one formto another in order to effect a desired state, and which changes one ofits properties—chemical, mechanical, optical, magnetic or thermal—inresponse to a change in the conditions in its environment.

Boats intended for use with 300 mm wafers will most likely be movedusing automated equipment which contacts the underside of the boat.Based on this type of boat transfer, the design was analyzed todetermine stress due to loading. The total possible wafer capacity ofthe boat was defined as 25 wafers, (approximately 3.38 kg). A safetyfactor of ten was chosen. Although this factor of safety is high,experience has shown that wafer boats are often exposed to severehandling. The brittle nature of ceramic materials and the interface ofthe boats with automation and rigid fixturing require a high safetyfactor.

As noted previously, the wafer boats of the present invention areintended to be used with 300 mm wafers. Of course, the present inventionis not intended to be limited strictly to wafers of this size. Rather,the inventive wafer boats are intended to be used both with existingsmaller wafer geometries, as well as with larger wafer geometries whichmay be developed.

In the case of a wafer boat for use with a 300 mm wafer, in oneembodiment, the boat includes 10 slots intended to hold 10 wafers. Sucha boat is approximately 11 cm long. The opposing upper supports arepositioned approximately 6.8 cm above the lowest point of the groove,and spaced apart from one another by approximately 10.4 cm. Each slotwill have a width of approximately 0.89 mm. The groove will have an arclength of approximately 20.82 mm. FIG. 4 depicts a triangle having ahypotenuse “A” defined as the wafer radius extending from the center ofthe wafer to the wafer periphery or edge at a point at which one of theupper support guides holds the wafer in the slot. An angle α definedbetween the hypotenuse “A” and a radius “B” originating at the center ofthe wafer and extending downward to a point on the wafer periphery whichis positioned in the middle of the grooved portion on which the waferrests. A third radius “C” extends from the center of the wafer to theperiphery of the wafer to a point at which the second upper supportingguide holds the wafer in the slot. In the inventive wafer boat, theangle α is in the range of 10 degrees to 80 degrees, and optimally about37 degrees. The total angle defined between radius A and radius C isapproximately 74 degrees.

It should be understood that each of the dimensions provided above isgiven at room temperature and is intended for wafer boats formed ofCRYSTAR® recrystallized SiC. The dimensions may differ for boats formedof different materials based upon the thermal expansion coefficients ofthose materials. Likewise, the dimensions will be different at waferprocessing temperatures of between approximately 1000° C.–1400° C.

EXPERIMENTAL

The wafer boat of the present invention was tested according to normalmethods of usage and the following results were obtained:

-   -   150 wafers were tested. None displayed any slip lines.    -   1 wafer had very faint slip lines detectable using a Hologenix        magic mirror.    -   2 wafers had scratches or very faint slip lines visible with the        Hologenix magic mirror.    -   The results compare to 20% slip lines obtained using standard        conti wafer boats.

EQUIVALENTS

From the foregoing detailed description of the specific embodiments ofthe invention, it should be apparent that a novel wafer boat has beendescribed. Although particular embodiments have been disclosed herein indetail, this has been done by way of example for purposes ofillustration only, and is not intended to be limiting with respect tothe scope of the appended claims which follow. In particular, it iscontemplated by the inventors that various substitutions, alterations,and modifications may be made to the invention without departing fromthe spirit and scope of the invention as defined by the claims.

1. A wafer boat combination comprising a wafer boat and a plurality of asemiconductor wafers, the wafer boat comprising: a first end and asecond end; a plurality of slots positioned between the first and thesecond ends, the semiconductor wafers being respectively received in theplurality of slots, each of the plurality of slots comprising first andsecond upper support guides to maintain the semiconductor wafers in avertical orientation; and a lower grooved portion for supporting aplurality of wafers, the wafer boat having an inner radius originatingfrom a centerpoint, the slots extending generally along an arc having aradius of curvature corresponding to the inner radius, wherein the lowergrooved portion has a generally arcuate and concave contour as viewedfrom the centerpoint, and wherein at semiconductor processingtemperatures of between approximately 1000° C. to 1400° C., the lowergrooved portion substantially conforms to the portion of the wafersupported thereon.
 2. The wafer boat combination of claim 1, wherein thewafer boat comprises silicon carbide.
 3. The wafer boat combination ofclaim 2, wherein the silicon carbide is recrystallized silicon carbide.4. The wafer boat combination of claim 1, wherein semiconductor wafershave a diameter of about 300 mm.
 5. The wafer boat combination of claim1, wherein an angle α in the range of 10–80 degrees is defined between(i) a first radius of a wafer of the plurality of wafers extending fromthe center of said wafer to the periphery of said wafer proximate thefirst upper guides, and (ii) a second radius extending verticallydownward from the center of said wafer to a point on the periphery ofthe wafer which corresponds to the center of the lower portion.
 6. Thewafer boat combination of claim 5, wherein the angle αis approximately37 degrees.
 7. The wafer boat combination of claim 1, wherein theplurality of slots between the first and second ends of said boat areconfigured to support up to 25 semiconductor wafers.
 8. The wafer boatcombination of claim 1, wherein the wafer boat has a thickness of notless than 5 mm.
 9. The wafer boat combination of claim 1, wherein theplurality of slots are spaced apart from each other at a constantspacing, such that the plurality of wafers are spaced apart from eachother at said constant spacing.